Gear actuated variable resistor



March 1966 v. G. MATHISON 3,242,451

GEAR ACTUATED VARIABLE RESISTOR Filed Feb. 18, 1964 2 Sheets-Sheet 1 INVENTOR.

United States Patent 7 3,242,451 GEAR ACTUATED VARIABLE RESISTOR Victor G. Mathison, Riverside, Califi, assignor to Bourns, Inc., a corporation of California Filed Feb. 18, 1964, Ser. No. 345,624 8 Claims. (Cl. 338-474) The invention hereinafter disclosed pertains to variable resistors, and more particularly to improvements in a variable resistor of the type comprising an arcuate resistance element disposed in a closed housing and wiped by a rotary contact device that is driven or rotated by a suitable driving device such as gearmeans. Variable resistors of the noted type or class are commonly referred to as single turn resistors. One species of single-turn variable resistor is that classed as worm gear actuated? as a consequence of the utilization of worm gear means for rotating the variable contact or wiper; The present invention, while applicable in certain respects to other species of single-turn variable resistors, is principally and in some respects specifically applicable to variable resistors of theworm gear actuated species, and accordingly will be disclosed in connection with a preferred exemplary physical embodiment as incorporated in a worm gear actuated variable resistor adapted for use as either a rheostat or a potentiometer. Typical prior-art variable resistors of that type are illustrated in US. Patents Nos. 2,945,198, 2,887,554, 3,054,077, 3,096,498, 3,099,810, and 3,108,245.

Briefly, the invention comprehends means whereby the number and complexity of parts or components found in resistors of the class here of concern are reduced, and the invention comprehends parts or components, and arrangements of the components, such that costs of the components and the expense of assembling them to form a complete variable resistor are considerably reduced. In certain respects the components are designed so that steps of assembly may be performed by machine. These advances the invention attains in part by forming the case or housing of the instrument of two patrs of molded material such as insulative synthetic resin, the two parts being in the form of interfitting base and cover members one of which is devised to receive and hold the arcuate resistance element, an electrical return device or collector, and terminal means, and the other of which parts is devised to receive and hold a novel rotary contact device, and gearing means for driving the contact device. The contact device is so devised that While it is in the form of a single piece of material, it performs the several functions of a clutch element,'a clutchspring, a contact means effective to wipe upon and electrically interconnect a selected portion of the resistance element and the collector, and of performing the function of holding gear means in place in the housing. To those ends the contact device is formed from flat resilient conductive sheet material, pierced and shaped to form a disc-like clutch plate and collector brush and an integral resilient contact and spring arm, the clutch plate portion being dimensioned and arranged to lit in friction engagement in and witha complementary annular recess in a gear of the gear means, with a central resilient collector brush portion adapted to provide multiple brushing contacts with a collector memher, and the contact limb forming with the clutch plate a V-shaped spring and integral contact adapted to wipe on the resistance element of the resistor, Conveniently, the contact device includes a struck portion disposed outward ly of the general plane of the clutch plate portion and adapted to rotate in'a groove formed in the housingeand' to be stopped by a formation of the housing whereby rotation of the contact is restricted to less than one full revolution. To facilitate assembly and to reduce the number of parts required, the cover and the base are formed to provide on each a respective coaxial circular post or member, on one of which the collector is disposed and on the other of which the noted gear of the gear means is rotatably mounted. By those means, the resistance element, collector device, and the terminal connections therefor may be fixedly mounted in a first of the housing members, and the gearing means and multiple-function contact device installed in the second housing member, both of the subassemblies then easily subjected to operating and electrical tests while open and with components accessible, and the first housing member and aflixed components then juxtaposed and adhesively unitedv With the other to complete the assembly of the instrument. The collector device may be a simple stamping of resilient material formed with a set of clamping legs adapted to grip the circular post and adapted to have welded thereto, alternatively, a wire terminal lead, or a pin terminal memher.

As has now been made evident or as will be more fully explained hereinafter, the single integral contact device serves as a clutch element, a stop or rotation-limiting element, a clutch spring, a gear retainer, a resilient brush or contact for brushing the resistance element, a resilient multipoint contact for the electrical tap or return terminal connection, and a contact spring for maintaining electrical contact with the resistance element and with the collector terminal.

The preceding brief general description of the inven tion makes it evident that a principal object of the invention is to provide patentable improvements in a single: turn variable resistor.

Another object of the invention is to provide improvements in contact devices for single-turn variable resistors.

Another object of the invention will hereinafter be made evident or set out in the appended claims or in the following description of I a preferred exemplary physical embodiment of the invention that is illustrated in the accompanying drawings forming a part of this specification or description, and in which drawings:

FIGURE 1 is a face or plan view of the noted exemplary variable resistor, depicted to a greatly enlarged scale and in the form of a potentiometer;

FIGURE 2 is a plan view of the base portion of the housing of the potentiometer depicted in FIGURE 1, with certain parts and a fragment of a part, showing dispositions of formations and elements of the driving mechanism of the potentiometer;

FIGURE 3 is a plan view similar to FIGURE 2 but showing one subassembly of the potentiometer;

FIGURE 4 is a plan view of the lower or inside face of djie Igover member of the potentiometer depicted in FIG- FIGURE 5 is a view similar to FIGURE 4, but with components assembled to form a cover subassembly, with a fragment broken away for purposes of illustration;

FIGURE 6 is an exploded sectional view of components of the potentiometer, illustrating relative dispositions of certain parts;

FIGURE 7 is a plan view of the contact device of the potentiometer, in relaxed condition;

FIGURE 8 is a view in elevation of the contact device depicted in FIGURE 7;

FIGURE 9 is a bottom face view of the contact device depicted in FIGURES 7 and 8;

FIGURE 10 is a bottom face view of a resistance element assembly depicted in place in'the cover in FIG- URE 5;.

FIGURE 11 is a view in elevation of a gear and clutch element comprised in the potentiometer depicted in FIG- UlllE 1,dwith a section removed to show cross-section detai s; an

FIGURE 12 is a plan view of a fragment of the gear shown in FIGURES 3 and 11.

Referring first to FIGURE 1, the exemplary singleturn variable resistor 20 is a worm gear adjusted potentiometer comprising a housing composed principally of a base 22 and a cover 24, both preferably formed by molding. The base 22, as indicated also in FIGURE 2, is of generally rectangular configuration and shaped to provide spaces such as 22i for reception of mounting screws, and also shaped to provide a beveled corner 22s for exit of terminal wires. Further the base is provided with an upstanding outer wall 22w that extends nearly all the way around a generally circular recess or cavity 220 (FIG- URE 6) in which cavity the cover 24 and certain operafive parts of the potentiometer are received. The discontinuous upstanding wall 22w provides a gap or passage 22p (FIGURES 2 and 6) through which terminal leads or conductors t1, t2 and t3 (FIGURES 1 and 6) extend; and the gap or passage is further formed to receive a terminal-retainer wing 24w that is an integral part of cover 24, as indicated also in FIGURES 4 and 5. The cover 24, as indicated in FIGURES 1, 4, 5 and 6, is formed to fit into the recess of base 22, being provided with a rim portion 24-1 that fits in and rests on a step or shoulder 22i formed in the interior wall of the base. Thus, as is indicated in FIGURE 1, the cover fits closely in the conforming outline of the base and with the latter provides a protected enclosure or chamber in which components are housed in a manner presently to be made clearly evident. The next-adjacent beveled outer edges of the cover and base provide a groove 26 (FIGURE 1) that is adapted to receive an adhesive resin sealant. As the last stage or procedure of assembly of the instrument, such a sealant (epoxy resin, for example) is applied around the exterior juncture of the cover and base, and the sealant is cured.

The base 22, as indicated in FIGURES 2 and 6, is provided with an annular formation 22b that rises above the level of the inner floor surface 22 of the base. The floor 22 and formation 22b form or provide annular bearing surfaces for supporting a driven member 28 for rotation on the floor and the formation, about an axis that is perpendicular to the lower outside surface 22z of the base. Member 28 is formed as a shaped ring gear and clutch element, having a set of gear teeth 282. (FIGURES 3, 11 and 12) formed on its exterior periphery, and having an annular frictional driving surface 28s provided as a step at the top face thereof as indicated. The inner cylindrical surface 28b of member 28 is dimensioned to fit on and about the upstanding formation 22b of the base for easy rotation about the bearing provided by that formation; and the lower annular face 28 of the member is such as to bear against and rotate on floor 22 of the base 22.

Formed in the upstanding wall of the base 22 is a bore 22g which communicates at its inner end with the chamber 22c. The bore 22g is complementary to and receives for rotation therein a driving gear or worm 30 (FIGURE 2), the inner end of which is threaded with a screw thread 301 as shown. The Worm is thus journaled for rotation in bore 22g, and is retained against axial movement therein by a retainer pin 32 that is driven into a bore formed in the wall of the cover to intersect the edge of bore 22g, and which pin engages the Worm at a circumferential groove 30g, formed around the worm as shown in FIG- URE 2. Preferably but not necessarily, the bore 22g may be sealed against ingress of moisture or passage of foreign material, by a ring-seal 34 that is disposed in an annular enlargement or groove 22x at the outer end of bore 22g adjacent the driving head 30h of the worm.

The dimensional characteristics and arrangement of worm 30 and member 28 are such that rotation of the worm causes rotation of member 28, while the latter is locked against rotation by the worm when the worm is not rotated or driven. Seated on the frictional driving surface 28s of the ring gear member 28 is a contact device 36 (FIGURES 6, 7, 8 and 9) the flat base 36b of which is formed with an extensive arcuate outer peripheral edge 36]) so dimensioned as to provide a sliding fit in the annular step forming surface 28s of member 28. Thus when device 36 is pressed against surface 28s, rotation of member 28 is transmitted f-rictionally to device 36; but slipping readily occurs if device 36 is arrested or held from rotating. Also, such pressure or force applied to the contact device 36 serves to hold gear member 28 against floor 22 of base 22. The contact device, reposing on the ring gear member 28, and the gear member engaged with worm 30 installed in base 22 as previously noted, forms with the noted members a subassembly of components that is illustrated in section at the lower portion of FIGURE 6 and in plan view in FIGURE 3.

The housing cover member 24 (FIGURES 4 and 6) is provided at its center with a cylindrical stud 24s outstanding or depending in a shaped recess 24m. The recess comprises a generally annular groove 242, and a somewhat rectangular depression or well 2411 next adjacent to the aforementioned terminal retainer wing 24w; The well communicates directly with a shaped slot 24p that is formed in retainer wing 24w of the cover 24. Seated in the groove 24z as is indicated in FIGURES 5 and 6, and secured therein by adhesive, is an arcuate resistance element structure 38. Structure 38 comprises an arcuate resistance element 38c and termination ta'bs 3 8t, 38t. As depicted, the resistance element is an arcuate core bearing a winding of resistance wire; however, any of other known forms of resistance elements of arcuate sh ape may be utilized. The termination tabs 38: and 38f are shaped flat conductive strips, each electrically connected to a respective end of the resistive material of of element 38e, as by welding, brazing, or the like. As is indicated in FIGURES 6 and 10, tab 38 has conductively secured thereto the end of the conductor of terminal lead t2, as by welding; and similarly the condoctor of terminal lead t1 is secured to tab 38f. The terminal leads t1 and t2, together with a return or collector terminal lead t3 presently to be explained, extend through and are adhesive'ly sealed in the aforementioned slot 24p in terminal-retainer wing 24w of the cover, as illustrated in FIGURE 5.

Secured on the post or stud 24s (FIGURE 6) of the cover 24 is a conductive return member or collector 40. The collector is cfiormcd with a plurality of fingers 40w (FIGURE 5) that preferably are res-ilent and grip the stud, but which may alternatively be adhesively secured to the stud. The fingers are integral wings or extensions from a disc-like but-ton portion that is disposed on the end of the stud. The button portion is formed with a round pimple or protuberance 40p at its center, to serve as a contact to be brushed by portions of contact device 36. The conductor of terminal lead t3 is welded to one of the fingers 40w prior to installation of the collector on studs 24s. Thus it is now evident that a coherent sub-assembly is formed by cover 24, structure 38, collector 40, and terminal loads 21, t2 and 13. The electrical characteristics of the element 3'82 and the terminals are easily determined by tests, and inspection is facilitated, by the nature of the subassembly.

Referring again to FIGURES 7, 8 and 9, the disc-like base portion 36b of contact device 36 is integral with a folded or spring-hinge lpontion 36h that in turn is integral with a ring-shaped extension 36r that serves to sup port an integral wiper arm 36w that is provided with a conductive contact 360 either by means of an applied pellet or by deformation of a portion of the arm, as depicted. Extention 36r serves to stabilize the contact against oscillation due to vibration to which the instrument may be subjected, provides clearance for collector 40, and acts as a cantilever member for transmitting force provided by hinge spring 3611 when the latter is stressed.

' The central part of base 36b of the contact device is pierced to provide-a plurality (three, as shown) inwardly extending resilent spring sectors such as 36p terminated by arcuate inner edges 36e that are disposed to make resilient brushing contact with the protuberance 40p of the collector'40. The con-tact device 36 is formed from and composed essentially of conductive resilient material, for the mentioned reasons or purposes.

The preceding description makes it evident that with the collector 40 on stud 24s and with the base 22 and cover 24 disposed as indicated in FIGURE 6, the two subassemblies may be brought'together with contact 360 contacting element 3*8e, the spring hinge 36h being stressed in the action, and edges 36e being brought into conductive resilient contact with'collector 40. With the cover adhesively secured to base 22, rotation of worm 30 by a tool applied to head 3011 will cause rotation of the ringgearmernber 28 and the frictional rotation of contact device 36 induced by fiorce applied to'lbase 36b of the contact device bythe stress created in hinge portion 36h of the contact device; Contact 360 will be maintained in contact with element 38e' bythe same spring action; and by suitable rotation of the Worm the electrical resistance exhibited between the conductor of terminal lead t3 and either of the other terminal lead conductors may be brought to a desired value; 1

To obviate driving of the contact 36c off either end of element 3'8e, suitable stop means are preferably provided. Thus 22 depending stop limb 36s (FIGURES 6, 8 and 9) is provided on contact device 36, and a complementary stop abutment 22v (FIGURE 2)' is provided at theends of an arcuate slot 22m forming part of the floor formation of base 22. Therel ative dispositions of stop limb 36s and contact 36c, and of stop abutment 22v and the ends of element 3'8e, are such that contact device 36 may be driiven to bring contact 360 to either electrical end of the resistive means of the resistance element, but not therebeyond. Hence the stop means precludes driving the contact 36c off the resistance elementand thus o Wiates open-ing the circuit between the collector and the resistance element. i

In the preceding description a certain element has been termed a cover, and another element has been termed a base; however, it is evident that either may be a base and the other a cover, depending solely on definitions of those terms.- Hence the terminology employed is not intended to be restrictive. It is evident from the description of the preferred exemplary physical embodiment of the invention that the contact device 36 serves the multiple [functions stated in the specification, that by utilizing the particular configurations and arrangement for components the variable resistor is made simple and easy to assemble, and that the parts are simple, few in number, and inexpensive yet may be produced with great precision and accuracy, whereby notable meritorious improvements over the known prior-art variable resistors of the mentioned class are secured and" the several stated objects of the invention are attained. It is evident that in the light of the present disclosure modifications and changes within the true spirit and scope of the invention will occur to others and hence it is not desired to limit the invention to the exact details and features of the illustrative exemplary embodiment shown and described, except as it maybe restricted by the appended claims. I claim: i

'1; A gear actuated variable resistor comprisingz first means, including a generally rectangular housing of box-like configuration comprising a base member and a cover member interfitted to provide an enclosed chamber, said base providingin' said chamber an annular bearing means;

second means, including gearing means including a driven gear disposed for rotation about said bearing means in said chamber and a driving gear in mesh with said driven gear and accessible from the exile rior of said housing, said driven gear including an annular friction face;

third means, including an arcuate resistance element in said chamber and disposed substantially about the axis of said bearing and afiixed to said cover, said third means comprising a fixed conductive collector disposed on said cover in said chamber and about the axis of said bearing, and terminal means for said resistance element and said collecton'and' a one-piece resilient contact device having a base portion having a surface in frictional engagement with said friction face and having a stressed resilient spring hinge portion merging with said base portion and a ring portion extending from said spring hinge portion and encircling the axis of said bearing and a wiper arm extending from said ring portion with a contact on said wiper arm resiliently urged by said spring hinge portion into wiping contact with said resistance element, said base portion having a plurality of resilient inwardly extending limbs each in conductive wiping engagement with said collector, said hinge portion being stressed toward closed condition,

whereby rotation of said driving gear induces rotation of said driven gear on said bearing and the latter frictionally drives said contact device to move said contact along said resistance element and whereby said driven gear is held in place on said friction face and said contact device provides a plurality of conductive paths between said collector and said resistance element.

2. A variable resistor according to claim 1, said base com-prising a stop abutment-and said contact device 'comprising a stop limb movable in an arcuate path terminated at both ends by said stop abutment, whereby rotational movement of said contact is restricted to movement along said resistance element. i

3. A worm gear actuated variable resistor comprising:

first means, including first and second interfitting housing members interfitted to provide a box-like housing providing an enclosed chamber, one of said members comprising a stud projecting into said chamber and the other of said members comprising a bearing means;

second means, including gearing means, including a driven gear rotatably disposed on said bearing means in said chamber and having an annular clutch sur face, said gearing means including a driving gear in mesh with said driven gear;

third means, including an arcuate resistance element disposed in said chamber, a conductive collector mounted on said stud, and'terminal means for said resistance element and said collector and extending therefrom to a location accessible from the'exterior of said housing; and

fourth means, consisting essentially of a contact device comprising a folded sheet of resilient conductive material having a flat base portion complementary to and engaging said annular clutch surface, saidbase portion comprising a'shaped aperture bounding a plurality of inwardly-extending spring limbs each disposed in'conductive contact with said collector, said contact device having a spring-hinge portion interconnecting said base portion and a resilient extension, said extension, diverging from said base portion at an acute angle thereto and overlying said base portion, said resilient extension including a contact disposed in electrical cont-act with said resistance element and thereto urged by said spring hinge portion, said hinge portion being stressed to oppositely force said flat base portion and said contact into firm contact with said annular clru-toh surface and said resistance element, respectively, while maintaining contact between said limbs and said collector.

4. A worm gear actuated variable resistor comprising:

first subassembly means, including a first housing member having a flat exterior face and an interior face with an outstanding stud, an arcuate resistance element secured to said member substantially encircling said stud, a conductive collector afiixed to said stud, and terminal means electrically connected respectively to said collector and said resistance element; second subassembly means, including a second housing member having a flat exterior surface and shaped and arranged to interfit with said first housing member and having an annular bearing surface, said second subassembly means including a driven gear having a clutch surface and mounted for rotation on said bearing surface and a driving gear mounted on said second housing member and meshing with said driven gear to drive the latter, said second housing member cooperating with said first housing member to form an enclosed chamber housing said collector, driven gear, and resistance element, and a contact device comprising a resilient folded sheet of conductive spring material having a base portion and an overlying other portion connected thereto by a spring hinge portion and arranged to be compressed between said driven gear and said resistance element, said contact device having its base portion complimentary to and engaging said clutch surface to be frictionally driven by said driven gear, said contact device including a contact on said overlying other portion arranged for wiping on said resistance element and said base portion having a plurality of contacts arranged for wiping on said collector for providing an electrical connection between the collector and the element, whereby incident to assembly together of said first and second subassembly means said folded sheet is compressed between said driven gear and said collector and said contact is brought into firm engagement with said resistance element to provide said electrical connection and to provide a friction-clutch drive between said driven gear and said contact device.

5. A variable resistor according to claim 4, in which said first and second housing means comprise stop abutment means protruding into said chamber and in which said folded sheet comprises a stop limb rotatable therewith in an arcuate path terminated by said stop abutment means. 7

6. In a worm gear actuated variable resistor having a housing comprising a driving worm gear and shaped to provide an enclosed chamber with an annular bearing on one face of the chamber and a protruding stud on the other face coaxial with the bearing, and having a collector on said stud and an arcuate resistance element in the chamber, the improvement comprising:

an integral gear and clutch member dimensioned to be rotatably mounted on the bearing in the resistor housing and having an annular frictional clutch surface, :and said gear and clutch member adapted and arranged to be driven by said worm gear, and

a contact device of resilient conductive sheet material having a fiat base port-ion comprising a pierced apertured center portion providing a plurality of spring cont act surface-s positioned to brush on the collector of the resistor and said base portion comprising an arcuate frictional clutch surface complementary with and frict-ionally engaging said annular clutch surface, and said contact device having a resilient extension disposed divergent to said base portion and overlying the same and connected to the latter by an integral a cover member complementary to said base memberl and with the latter member providing a housing de-g fining an enclosed chamber; a

a gear rotatably mounted on said annular bearing means and having an annular friction clutch surface, and a Worm rotatably mounted on said housing in mesh with said gear for rotating the latter;

an arcuate resistance element mounted on said housing in said chamber, and terminal connections extending from said element to a location accessible from the exterior of said housing;

a rotary device having an outer rim portion presenting a friction surface complementary to said annular friction clutch surface and engaged therewith, said rotary device comprising a contact and a resilient stressedfolded portion overlying said outer rim portion and carrying said contact and acting to press said contact into engagement with said element and said friction surface into engagement with said annular friction clutch surface, said rotary device comprising integral with said rim portion, a portion providing a plurality of inwardly directed resilient conductive limbs; and v a collector affixed to said housing in said chamber and disposed in electrical contact with said conductive limbs. 8. A rotary contact device for a worm gear adjusted variable resistor, comprising:

an acutely folded resilient sheet having a generally disc-shaped base portion and an extension and an integral resilient spring-hinge portion interconnecting said base portion and said extension with said extension disposed divergent from said base portion and having a ring portion overlying the base portion and having a contact, said base portion comprising an arcuate portion presenting a friction clutch surface along an outer area thereof and said base portion having a pierced and apertured central region forming a plurality of resilient inwardly extending limbs integ-r-al'with said arcuate portion, said springhinge portion being adapted to be stressed to apply force on said ring portion and on said base portion in respective opposite directions.

References Cited by the Examiner UNITED STATES PATENTS 2,098,273 11/1937 Bradley et a1 338--202X 3,124,778 3/1964 Youngbeck 338-175 3,195,092

7/1965 Tumbusch 338148 RICHARD M.- WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner. 

1. A GEAR ACTUATED VARIABLE RESISTOR COMPRISING: FIRST MEANS, INCLUDING A GENERALLY RECTANGULAR HOUSING OF BOX-LIKE CONFIGURATION COMPRISING A BASE MEMBER AND A COVER MEMBER INTERFITTED TO PROVIDE AN ENCLOSED CHAMBER, SAID BASE PROVIDING IN SAID CHAMBER AN ANNULAR BEARING MEANS; SECOND MEANS, INCLUDING GEARING MEANS INCLUDING A DRIVEN GEAR DISPOSED FOR ROTATION ABOUT SAID BEARING MEANS IN SAID CHAMBER AND A DRIVING GEAR IN MESH WITH SAID DRIVEN GEAR AND ACCESSIBLE FROM THE EXTERIOR OF SAID HOUSING, SAID DRIVEN GEAR INCLUDING AN ANNULAR FRICTION FACE; THIRD MEANS, INCLUDING AN ARCUATE RESISTANCE ELEMENT IN SAID CHAMBER AND DISPOSED SUBSTANTIALLY ABOUT THE AXIS OF SAID BEARING AND AFFIXED TO SAID COVER, SAID THIRD MEANS COMPRISING A FIXED CONDICTIVE COLLECTOR DISPOSED ON SAID COVER IN SAID CHAMBER AND ABOUT THE AXIS OF SAID BEARING, AND TERMINAL MEANS FOR SAID RESISTANCE ELEMENT CONTACT DEVICE HAVING A BASE PORA ONE-PIECE RESILIENT CONTACT DEVICE HAVING A BASE PORTION HAVING SAID FRICTION ENGAGEMENT WITH SAID FRICTION FACE AND HAVING A STRESSED RESILIENT SPRING HINGE PORTION MERGING WITH SAID BASE PORTION AND A RING PORTION EXTENDING FROM SAID SPRING HINGE PORTION AND ENCIRCLING THE AXIS OF SAID BEARING AND A WIPER ARM EXTENDING FROM SAID RING PORTION WITH A CONTACT ON SAID WIPER ARM RESILIENTLY URGED BY SAID SPRING HINGE PORTION INTO WIPING CONTACT WITH SAID RESISTANCE ELEMENT, SAID BASE PORTION HAVING A PLURALITY OF RESILIENT INWARDLY EXTENDING LIMBS EACH IN CONDUCTIVE WIPING ENGAGEMENT WITH SAID COLLECTOR, SAID HINGE PORTION BEING STRESSED TOWARD CLOSED CONDITION, WHEREBY ROTATION OF SAID DRIVING GEAR INDUCES ROTATION OF SAID DRIVEN GEAR ON SAID BEARING AND THE LATTER FRICTIONALLY DRIVES SAID CONTACT DEVICE TO MOVE SAID CONTACT ALONG SAID RESISTANCE ELEMENT AND WHEREBY SAID DRIVEN GEAR IS HELD IN PLACE ON SAID FRICTION FACE AND SAID CONTACT DEVICE PROVIDES A PLURALITY OF CONDUCTIVE PATHS BETWEEN SAID COLLECTOR AND SAID RESISTANCE ELEMENT. 